While the non-Steam game may be hard to find nowadays, this is not true for the Patch 1.2.0. So if you have version 1.0 of the game (which everyone does when having the DVD-PC version) the question is: Can you patch it? With a non-Steam the most likely answer is: YES. So you should do fine with a non-Steam 1.0.
Here is 1 among the first google results:
http://www.gamestar.de/artikel/silen...d,2314914.html